Method for determining the moment of closure of a circuit breaker on a high voltage line

ABSTRACT

A method for determining the reclose time of a circuit breaker on a three-phase high-voltage electric network after separation of contacts  7 A,  8 A,  7 B,  8 B,  7 C,  8 C in the presence of a fault on one of the three phases A, B or C includes measuring voltages UL A0 , UL BO  and UL CO , measuring voltage US AO ,determining the voltage US A0 , US B0 , and US CO , calculating the differences UL AB , UL AC  and UL BC , calculating the differences US AB , US AC , and US BC . From these measurements and calculations, a determination of the reclose time is made on the basis of the voltage differences.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on International PatentApplication No. PCT/FR2004/050137, entitled “Method for Determining theMoment of Closure of a Circuit Breaker on a High Voltage Line” byJean-Pierre DUPRAZ, Houria SIGUERDIIDJANE, Farah BOUDAOUD and PatrickBASTARD, which claims priority of French Application No. 03 50086, filedon Apr. 2, 2003, and which was not published in English.

BACKGROUND OF THE INVENTION/FIELD OF THE INVENTION

The present invention concerns a method for determining the reclose timeof a circuit breaker on an electric network comprising a three-phasetransmission line and a high voltage source. The method of the inventionis more particularly adapted to extra high voltage lines, i.e. with anominal source voltage of several hundred kV.

Such an electric network can be modelled according to a firstapproximation by the equivalent circuit shown FIG. 1. The network 1comprises:

-   -   a voltage source S,    -   a three-phase transmission line L,    -   a circuit breaker 6,    -   a shunt compensation reactor 5,    -   a capacitive voltage transformer 4.

The source voltage S is a very high voltage having a nominal value of500 kV for example and a network frequency of 50 Hz.

The three-phase transmission line L is a line of length 400 km forexample enabling the transport of three phases A, B and C.

By circuit breaker is meant both a circuit breaker commanded by threeindependent single-pole commands each associated with a phase, and acircuit breaker commanded by a single three-pole command. Generally thethree-phase circuit breaker 6 comprises at least three cut-off chamberseach associated with one of phases A, B or C. If nominal voltage ishigh, several cut-off chambers may be connected in series. The circuitbreaker 6 therefore comprises at least three pairs of contacts, eachpair being associated with one of the three phases of line L and makingit possible to interrupt any current circulating between source S andline L by separating the two contacts in the event of a fault on theassociated phase, the first contact being on the source side and thesecond contact being on the line side. Only two contacts 7 and 8associated with a phase of the circuit breaker 6 are shown in FIG. 1.

The shunt compensation reactor 5 is an inductance coil for exampleallowing compensation of capacitive reactive power on long,high-voltage, electric power transmission lines.

The capacitive voltage transformer 4 located at the start of the line isused to measure voltage on the line side of the circuit breaker.

A sudden change in configuration of the energy transport networkgenerated by the functioning of a circuit breaker causes a rapidtransient overvoltage, called switching surge which propagates over thenetwork. These switching surges may occur on the tripping or reclosingof circuit breakers. Since the use of circuit breakers withoutre-enabling has become generalised (i.e. with automatic re-setting ofthe circuit breaker after tripping) it is on closing and especially onreclosing a line still containing a trapped charge that the highestovervoltages occur.

A first solution to this problem consists of using an auxiliary systemcomprising a so-called closure resistance in series with a pair ofauxiliary contacts, said auxiliary system being mounted in parallel withthe cut-off chamber. The auxiliary contacts are actuated a few momentsbefore the contacting of the main contacts so as to insert the closureresistance in the circuit. With this two-step triggering it is possibleto reduce closure over-voltages with great efficacy.

This first solution, although very effective, has the drawback of beingvery costly.

A second solution consists of controlling the reclose times of thecircuit breakers using electronic synchronization devices to replace theclosure resistances. Said devices allow synchronized switching of a highvoltage transmission line.

Therefore, when a single phase fault occurs (which account for more than90% of line faults) on a high voltage line it is possible in certainnetworks that the elimination of the fault involves three-pole openingof the circuit breaker followed by almost immediate re-closing (between300 ms and 1 s) in the endeavour to obtain ensured continuity ofservice. In this case, two of the phases are therefore vacuum switchedby one of the end-of-line circuit breakers. On reclosing, the circuitbreaker contacts must be closed at the right time (i.e. substantially atthe time when the voltage at the contact terminals of the circuitbreaker is zero) on these two vacuum phases, so as to limit overvoltagesto an adequate value. This moment varies according to networkconfiguration and must be determined by a closure algorithm in relationto the voltage signals measured on the network and supplied to thealgorithm. The choice of right moment for reclosure is based on analysisof the voltage, at the contact terminals of the circuit breaker, of eachhealthy phase.

However, the implementing of this second solution also raises somedifficulties.

For example the lines compensated by shunt reactance have thecharacteristic, after opening, of oscillating at a frequency in theorder of 50 to 90% of the network frequency. This oscillation frequencyis chiefly related to line capacity and its shunt compensationreactance. The voltage occurring on the terminals of the circuit breakertherefore shows greater or lesser beats depending upon the extent ofcompensation which varies in relation to transmitted power. Duringsynchronization on reclosure, zero beat must be targeted to limitovervoltages. The determination of this zero beat is not easy insofaras, in practice, the faulty phase influences the signal of the twohealthy phases so that the signal obtained which is to be analysed atthe contact terminals of the circuit breaker of the two healthy phasesis of very complex shape and difficult to analyse. Consequently resultsmay not reach the desired accuracy of synchronization. Obtaining asatisfactory result i.e. at a determined confidence level, requires arelatively long convergence time for the closure algorithm used.

SUMMARY OF THE INVENTION

The present invention sets out to provide a method for determining thereclose time of a circuit breaker on an electric network, with which itis possible to improve the quality of the signals to be analysed andthereby to reduce the convergence period of the algorithm used fordetermining the reclose time of the circuit breaker contacts afterthree-pole tripping due to the presence of a single-phase fault.

For this purpose the present invention proposes a method for determiningthe reclose time of a circuit breaker on an electric network, saidnetwork comprising:

DESCRIPTION OF THE INVENTION

-   -   a high voltage source    -   a three-phase transmission line    -   a circuit beaker comprising at least three pairs of contacts,        each pair being associated with one interruption of any current        circulating between said source and said line by the separation        of said two contacts in the event of a fault on the associated        phase, the first contact being on the source side and the second        contact being on the line side,    -   a shunt compensation reactor to compensate capacitive reactive        power on said line,        said reclose time being determined in the event of separation of        the contacts of each pair of contacts in the presence of a fault        on one of the three phases, said determination of said reclose        time being performed using the following steps:    -   measuring the voltage between the line side contact and earth,        for each of the phases,    -   measuring the voltage between the source side contact and earth,        for at least one phase,    -   determining the voltage between the source side    -   contact and earth, for each of the phases,        said determination of said reclose time being characterized in        that it also comprises the following steps:    -   calculating, for two separate phases called first and second        phases, the voltage difference between the line side contact and        earth for said first phase, and the voltage difference between        the line side contact and earth for said second phase, the        calculation being made for each pair of separate phases,    -   calculating the voltage difference between the source side        contact and earth for said first phase and the voltage        difference between the source side contact and earth for said        second phase, the calculation being made for each pair of        separate phases,    -   determining said reclose time on the basis of said voltage        differences.

Under the invention, the algorithm for determining reclose time usesinput data corresponding to the difference in signals between two phaseson the source side and on the line side. With the method of theinvention it is therefore possible in particular to calculate thevoltage difference between two healthy phases on the line side and onthe source side. The faulty phase influences the signals of the twohealthy phases and, by using the difference in signals between twohealthy phases, it is possible to eliminate the influence of the faultyphase. In this way a signal is recovered whose shape is much simpler toanalyse. This makes it possible to use algorithms for determiningreclose time that have reduced convergence time for a given confidencelevel. It is necessary to calculate the difference for each pair ofseparate phases since it is not known in advance which phase is faulty.

Advantageously, said determination of the reclose time is made bycomparing said voltage differences between two healthy phases on theline side and source side.

Advantageously, said determination of said reclose time is made bydetermining the instant at which the two signals corresponding to thesaid voltage differences between two healthy phases on the line side andsource side are substantially equal and show the same monotony over anon-zero interval about said instant.

Advantageously, said determination of the voltage between the contact onthe source side and earth for each of the phases is made by conducting120° and 240° phase shifting of said voltage measured between the sourceside contact and earth for at least one phase.

Advantageously, said measurement of the voltage between the line sidecontact and earth for each phase is made using a voltage transformer.

Other characteristics and advantages of the present invention willbecome apparent in the following description of an embodiment of theinvention given as an illustrative example and in no way restrictive.

In the following figures:

FIG. 1 gives an equivalent circuit of an extra-high voltage electricnetwork

FIG. 2 schematically shows the three phases of an extra high voltageline such as the one in FIG. 1 on which the method of the invention isimplemented,

FIG. 3 shows two voltage signals obtained using the method of theinvention.

In all the figures, the elements in common carry the same referencenumbers.

FIG. 1 has already been described in connection with the state of theart.

FIG. 2 is a diagram of the three phases A, B and C of a high voltageline such as the line shown FIG. 1 on which the method of the inventionis implemented.

The circuit breaker 6 comprises three pairs of contacts (7A, 8A), (7B,8B) and (7C, 8C), each pair being associated with one of the threephases A, B or C and enabling the interruption of any currentcirculating between source S and line L through the separation of thetwo contacts in the event of a fault on the associated phase, the firstcontact being on the source side and the second contact being on theline side.

Let us assume that one of the phases A, B or C shows a fault; as aresult circuit breaker 6 is tripped for the three phases A, B and Cthereby separating the source side S from the line side L; it is saidthat the circuit breaker has three-pole tripping on a single-phasefault.

The objective of the method according to the invention is to provide thealgorithm determining the reclose time of circuit breaker 6 with datasignals that are easy to analyse. This algorithm must determine areclose time at which the voltages at the contact terminals of thecircuit breaker are substantially zero. The algorithm used may forexample be an algorithm using Prony's method, a neural recognitionalgorithm or a wavelet analysis algorithm such as described in the work“Wavelets and concurrent algorithms” (Y. Meyer, Hermann, 1992).

The method of the invention comprises a first step to measure thevoltages UL_(A0), UL_(B0) and UL_(CO) between the line side contacts ofphases A, B and C and earth. These three measurements are made usingthree capacitive voltage transformers, not shown, each associated withone phase.

In parallel, the method also comprises a voltage measurement step for atleast one phase A, B or C between the source side contact of this phaseand earth. Let US_(A0) be this voltage between the source side contactof phase A and earth.

The two other voltages US_(B0) and US_(C0) between the contacts forphases B and C and earth are deduced from voltage US_(A0) by 120° and240° phase shifting of signal US_(A0).

The method of the invention then comprises a calculation step of the sixfollowing differences:US _(A0) −US _(B0) =US _(AB)US _(A0) −US _(C0) =US _(AC)US _(B0) −US _(C0) =US _(BC)UL _(A0) −UL _(B0) =UL _(AB)UL _(A0) −UL _(C0) =UL _(AC)UL _(B0) −UL _(C0) =UL _(BC)

These six differences are firstly the three voltage differences betweentwo different phases on the source side, and secondly the three voltagedifferences between two different phases on the line side.

Let us assume that the initial fault concerns phase A.

Phase A, having a fault, influences the two healthy phases B and C. Thedifferences US_(B0)−US_(C0)=US_(BC) and UL_(B0)−UL_(C0)=UL_(BC) are usedto eliminate the effect of phase A on phases B and C, phase A acting insubstantially identical manner on the two other healthy phases B and C.In this way two signals are obtained of much simpler form to analysethan signals obtained directly through the differences US_(B0)−UL_(B0)and US_(C0)−UL_(C0); said differences remain disturbed by the faultyphase and provide very difficult signals for subsequent analysis.

It is to be noted that it is necessary to calculate the six differences,and not only the differences related to the two healthy phases, since itis not known in advance which phase is the faulty phase.

The six differences corresponding to signals US_(AB), US_(AC), US_(BC),UL_(AB), UL_(AC) and UL_(BC) are then provided as input data for thedetermination algorithm which will determine the time of the closurecommand of circuit breaker 6.

FIG. 3 therefore shows an example of voltage differences US_(BC)(=US_(B0)−US_(C0)) and UL_(BC) (=UL_(B0)−UL_(C0)) in relation to time inthe event of a fault on phase A. Signals US_(BC) and UL_(BC) are nearsinusoidal signals that are very scarcely disturbed and thereforerelatively easy to analyse by the determination algorithm.

One manner of determining closure time T consists for example of takingposition at a time when the two signals US_(BC) and UL_(BC) aresubstantially equal and show the same monotony over a non-zero intervalabout T. The two arrows shown in FIG. 3 give two examples of the choiceof time T (in the first case the two signals US_(BC) and UL_(BC)increase about T, and in the second case the two signals US_(BC) andUL_(BC) decrease about T).

Evidently the invention is not limited to the embodiment just described.

In particular the measuring, for each of the phases, of the voltagebetween the line side contact and earth has been described as being madeusing a capacitive voltage transformer but it may also be made using aninductive voltage transformer.

1. Method for determining a reclose time of a circuit breaker (6) on anelectric network, said network comprising: a high voltage source (S), athre-phase transmission line (L), a circuit breaker (6) comprising atleast three pairs of contacts (7A,8A ; 7B,8B; 7C,8C), each pair beingassociated with one of the three phases (A, B C) of said line (L) andallowing an interruption of a current circulating between said source(S) and said line (L) by separating said contacts in the event of afault on the associated phase, the first contact being on the sourceside and the second contact being on the line side, a shunt compensationreactor (5) to compensate capacitive reactive power of said line (L),said reclose time being determined in the event of separation of thecontacts of each pair of contacts in the presence of a fault on one ofthe three phases, said determination of said reclose time being madeusing the following steps: measuring a voltage (UL_(A0),UL_(B0),UL_(C0)) between the line side contact and ground for each of thephases, measuring the a voltage (US_(A0)) between the source sidecontact and ground for at least one phase, determining a voltage(US_(A0), US_(B0), US_(C0)) between the source side contact and groundfor each of the phases, said determination of said reclose time beingcharacterized in that it also comprises the following steps:calculating, for two separate phases called first and second phases, thevoltage difference (UL_(AB), UL_(AC), UL_(BC)) between the line sidecontact and ground for said first phase, and the voltage differencebetween the line side contact and ground for said second phase, thecalculation being made for each pair of separate phases, calculating thevoltage difference (US_(AB), US_(AC), US_(BC)) between the source sidecontact and ground for said first phase, and the voltage differencebetween the source side contact and ground for said second phase, thecalculation being made for each pair of separate phases, determiningsaid reclose time on the basis of said voltage differences.
 2. Methodaccording to claim 1 characterized in that said determination of saidreclose time (T) is made by comparing said voltage differences (US_(BC),UL_(BC)) between two healthy phases on the line side and source side. 3.Method according to claim 1, characterized in that said determination ofsaid reclose time (T) is made by determining the time at which the twosignals corresponding to said voltage differences (US_(BC), UL_(BC))between two healthy phases on the line side and source side aresubstantially equal and show the same monotony over a non zero intervalabout said time.
 4. Method according to claim 1 characterized in thatsaid determination of the voltage (US_(A0), US_(B0), US_(C0)) betweenthe source side contact and ground for each of the phases is made by120° and 240° phase shifting of said voltage (US_(A0)) measured betweenthe source side contact and ground for at least one phase.
 5. Method toclaim 1, characterized in that said measurement of the voltage betweenthe line side contact and ground for each phase is made using a voltagetransformer (4).